Metal tubes have many different applications across a broad spectrum of industrial uses. One example use of metal tubes is in heat exchanger configurations. Fluids or gases running through and over the tubes in the heat exchanger provide heating or cooling as desired. One such heat exchanger application is in the form of a condenser. A condenser is generally utilized to cool steam as it passes over the heat exchanger tubes, which have cooling water passing therethrough. Corrosion, deterioration, erosion, pitting, and fouling of condenser tubes can play a major role in the effectiveness of the heat exchanger apparatus. In addition, maintenance costs, water, chemistry, replacement costs, and down time for repair, are other issues that relate to the performance of the tubes in the condenser or heat exchanger.
The purpose of the tubes in heat exchanger configuration is to provide a barrier between the cooling media (in the form of water, most often) and the heated fluid, and to facilitate heat transfer. Over the course of time, the inner surfaces of the tubes can pit or erode, and eventually may begin to leak and cease to be an effective barrier.
In an effort to prevent or delay the formation of pits or erosion within the tubes, epoxy coatings and other rebuilding compounds have been used. In particular, coatings have been used to protect tube interiors of copper based alloys at the inlet end where water turbulence in conjunction with entrained solids can cause accelerated erosion damage. Coatings extending three inches to twenty-four inches into the tube have been successful in preventing degradation in this area.
In addition, more recent approaches have involved coating the entire length of the tubes. Since coatings often significantly reduce fouling and corrosion of the inner surfaces of the tubes, long term performance of coated tubes can ultimately be better than uncoated tubes. One potential side effect associated with the use of coatings is the extent to which heat transfer varies with different characteristics relating to the coatings. Various factors will affect how a coating affects heat transfer, such as but not limited to thermal conductivity of the coating, interface effects between coating and tube, interface effects between multiple coatings, laminar flow effects, fouling effects and applied thickness. The thermal conductivity of the coating is a factor of the resin and filler blend in addition to how well integrated the resin and filler blend are to the other. Interface effects are a function of coating wetability and application parameters, such as temperature, humidity, dust control, and number of coats. In addition, the applied thickness of the coating varies with the number of coats. More specifically, conventionally two coats have been applied to the interior portions of the tubes, however, one coat is preferable because of the reduced thickness and reduced material costs. A full length tube coating currently is typically applied using a spraying process resulting in a coating thickness on the order of 2 mils to 5 mils. Such a thickness can penalize heat transfer capabilities, reducing them in the range of 15%-38% before fouling factors are considered.
Once tubes are placed into service in a heat exchanger they develop protective oxide layers and begin to foul. If the fouling rate is rapid, then tube performance can degrade quickly. Depending on the design cleanliness assumptions and available capacity of tubes, such degradation of performance is tolerable to a certain extent until such time as the heat exchanger must be cleaned or the tubes ultimately replaced. Coatings can prevent formation of oxides and also reduce the rate at which fouling occurs.
A significant concern relating to the degradation of heat transfer characteristics and overall performance of heat transfer tubes relates to the effect of pin holes or pitting due to corrosion of the inner surface of the tube. Currently, common materials utilized for tubes include copper alloys, stainless steel alloys, and titanium alloys, and carbon steel. These tubes work by forming passive films in their intended service. When the passive film breaks down, corrosion occurs. Coatings placed on the inner surface of the tubes can obviate the need for a passivation layer to form.